The present invention relates to an automatic gain control (AGC) circuit for controlling gain of a video signal in a television receiver, the AGC circuit being applicable to an AGC operation of a video signal in which the offset processing is randomly applied to, for example, the sync signal for scrambling the video signal in pay television.
As is generally known, an AGC circuit maintains gain at a constant level. In a television receiver, for example, the AGC circuit detects the signal level of the video signal obtained by video detection. Based on the detected level, the AGC circuit controls the gain of an intermediate amplifier circuit (IF circuit) and that of a radio frequency circuit (RF circuit) in the tuner. The AGC operation for the IF circuit is called IFAGC, while that for the RF circuit is called RFAGC.
These two types of AGC operation, RFAC and IFAGC, are required in order to obtain a compromise between the noise figure and the cross-modulation distortion. In order to obtain a good noise figure, the greater the gain of the RF circuit, the better. However, a high-gain RF circuit tends to generate a distorted signal due to the nonlinearity involved in an amplifier in the RF circuit. This distorted signal causes cross-modulation interference. Thus, the RF circuit and the IF circuit must be subjected to the AGC control, in order to obtain satisfactory performance with respect to both the noise figure and the cross-modulation interference.
In general, in the amplifier, by a distinguished nonlinearity, the following relation holds between the input signal voltage and the output signal voltage of the amplifier. ##EQU1## where Y (e) is the output signal voltage of the amplifier
e is the input signal voltage of the amplifier PA1 Kn is the coefficient expressing the linearity of the amplifier PA1 n is the distortion factor
In the above relation (1), the nonlinearity distortion contains the distortion components of very high order. However, the distortion components, which must be considered in practical use, are those which are, at most, up to the secondary order (n=2) or the tertiary order (n=3).
When the distortion as given by expression (1) above is produced in a tuner, cross-modulation interference and beat interference occur. The degree of cross-modulation, which represents the degree of cross-modulation interference, is proportional to the square of the signal giving rise to the cross-modulation interference. The cross-modulation interference appears noticeably in a system having a number of transmission channels, such as a CATV.
Beat interference occurs when a number of high-frequency signals are concurrently transmitted, and also occurs if the distorted component generated in the amplifier is present within the frequency band of the high-frequency signal.
When RFAGC is performed on the RF circuit having a plurality of mixers, an RF amplifier is provided at the prestage of the first mixer. The gain of the RF amplifier is controlled in order to reduce distortion. This approach is satisfactory from a viewpoint of reducing the distortion, but is not good from a viewpoint of reducing the C/N (carrier-to-noise) ratio. The C/N ratio is generally given as EQU C/N [dB]=ei [dB.mu.]-NF [dB]-0.86 dB (2)
where ei is the input signal voltage of the amplifier, and NF is the noise figure. Let the number of cascade-connected amplifiers to be "m", and then the overall C/N ratio (C/N)m is given as EQU (C/N)m [dB]=(C/N) [dB]-10 log.sub.10 m (3)
The overall C/N ratio (C/N)m is inversely proportional to the number "m" of cascade-connected amplifiers. This means that when number "m" of amplifiers with the same performance are connected in a cascade fashion, the C/N ratio is degraded by 10 log.sub.10 m [dB].
Therefore, in order that the C/N ratio, when the amplifiers of "m" stages are cascade-connected, is kept at that for a single stage of amplifier, the input signal level ei at each amplifier is given as EQU ei [dB.mu.]=e min [dB.mu.]+10 log.sub.10 m (4)
where e min is the minimum signal level as calculated using expression (2) above.
As can be seen from the above relation, in order to obtain a predetermined C/N ratio, it is required that the signal be at a predetermined level. Therefore, the gain of an RF amplifier having a function for improving the C/N ratio must be kept at such a value as not to impair the distortion characteristics.
When "m" stages of amplifiers are connected in a cascade fashion, the overall noise figure NFt is given as ##EQU2## where G.sub.1, G.sub.2, . . . G.sub.m are the power gains of respective amplifiers, and NF.sub.1, NF.sub.2, . . . NF.sub.m are the noise figures of the respective amplifiers.
Expression (5) shows that the greater gain of the RF amplifier provides a satisfactorily low noise figure.
For the above reasons, the AGC operation is performed on the RF circuit and the IF circuit. The RF circuit is controlled so as to suppress the distortion, and the IF circuit is controlled so that the signal gain is kept constant.
In recent years, pay-television, such as CATV has been developed. Various types of protection systems have been incorporated in such pay-television system in order to prevent people other than the subscribers from watching the television. In one of the protection systems, a scrambled signal, for example, is used as a broadcast signal. By using a descramble circuit, provided in the subscriber's terminal, the scrambled signal is descrambled, and a normal video signal is obtained.
Various systems are used for scrambling the video signal. These include RF scramble and base band scramble. The base band scramble can be classified into the video insertion system, the sync offset system, and the sync suppress system.
Pay-television such as CATV receives a multi-channel broadcast signal. In this case, due to the nonlinearity distortion of the active elements in the RF amplifier and in the mixer for frequency-converting in the tuner, an unwanted distorted signal, as expressed by expression (1) above, is generated. The distorted signal, or cross-modulation distortion, affects the video signal. In order to prevent the effect by this distortion, the gain of the tuner is controlled.
The input field strength (AGC delay point) at which the RFAGC operation is initiated in consideration of cross modulation distortion level and the noise figure is determined. In other words, the field strength at which RFAGC operation is initiated, together with IFAGC operation, is determined. Thus, the gain of the input signal is suitably controlled.
As a result, optimal gain control of the input signal is performed by the IFAGC and RFAGC operations. However, the AGC operations have to follow up the change of the input level.
This requirement causes problems, especially when the AGC operation is performed in relation to a video scrambled signal whose horizontal sync signal is made offset. That is to say, full consideration has to be taken for the response characteristics of the IFAGC and RFAGC.